The most common and most likely answer to this crossword clue is the 4 letter word LICE
I've seen this crossword clue in Financial Times
L
I
C
E
Buddy explains!

The straight part
We know that "parasites" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case parasites = lice.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does LICE mean?
LICE noun- A small parasitic wingless insect of the order Psocodea.
- (not usually used in plural form) A contemptible person; one who is deceitful or causes harm.